Coulomb's Law (Electric Force) Video Lessons

Concept

# Problem:  Two point charges are separated by 6 cm (6 cm = 0.06 m). The attractive force between them is 20 N (20 Newtons). Find the force between them when they are separated by 12 cm (12 cm = 0.12 m). (Why can you solve this problem without knowing the magnitudes of the charges?) What do you know about the sign of the charges (positive, negative, etc)?

###### FREE Expert Solution

In this problem, we need to know Coulomb's law, which is expressed as:

$\overline{){\mathbf{F}}{\mathbf{=}}\frac{\mathbf{k}{\mathbf{q}}_{\mathbf{1}}{\mathbf{q}}_{\mathbf{2}}}{{\mathbf{r}}^{\mathbf{2}}}}$, where k is Coulomb's constant and r is the distance between the two charges, q1 and q2.

We're given,

r1 = 0.06 m, F1 = 20N, and r2 = 0.12 m

From Coulomb's law,

${\mathbit{F}}_{\mathbf{1}}\mathbf{=}\frac{\mathbf{k}{\mathbf{q}}_{\mathbf{1}}{\mathbf{q}}_{\mathbf{2}}}{{{\mathbf{r}}_{\mathbf{1}}}^{\mathbf{2}}}\phantom{\rule{0ex}{0ex}}{\mathbit{F}}_{\mathbf{2}}\mathbf{=}\frac{\mathbf{k}{\mathbf{q}}_{\mathbf{1}}{\mathbf{q}}_{\mathbf{2}}}{{{\mathbf{r}}_{\mathbf{2}}}^{\mathbf{2}}}$

79% (282 ratings)
###### Problem Details

Two point charges are separated by 6 cm (6 cm = 0.06 m). The attractive force between them is 20 N (20 Newtons). Find the force between them when they are separated by 12 cm (12 cm = 0.12 m). (Why can you solve this problem without knowing the magnitudes of the charges?) What do you know about the sign of the charges (positive, negative, etc)?